$ { {-1} \times \left[\begin{array}{r}{2} \\ {4} \\ {-1}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-1}\times{2} \\ {-1}\times{4} \\ {-1}\times{-1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-2} \\ {-4} \\ {1}\end{array}\right]}$